1. Field of the Invention
The present invention relates to a squirrel-cage rotor used for an induction motor, for example, and a production method thereof.
2. Description of the Related Art
For example, Japanese Unexamined Patent Publication (Kokai) No. 2011-10498 discloses a squirrel-cage induction motor having a rotor core, a large number of rotor slots formed at intervals near the outer periphery of the rotor core, rotor conductors embedded in the rotor slots, and conductive end rings arranged on both ends of the rotor core. The rotor core has end plates on both ends of electromagnetic steel plate, and the area of the rotor slots in the end plates is larger than the area of the rotor slots in the electromagnetic steel plate.
Japanese Patent Publication No. 4560755 discloses a production method for a rotor of an induction motor, wherein, a pair of end rings, radially extending along the outer periphery surface of a rotor core, are formed by thermal spraying. In the thermal spraying, a melted conductor such as melted copper is sprayed onto the outer periphery surface of the core at high speed. Then, the conductor is instantly cooled, whereby a conductive coating is formed on the outer periphery surface of the core. By means of the conductive coating, a conductive end ring is formed on the core.
In “Overview of cold spray technology and cold sprayed light metals coatings” (Journal of Japan Institute of Light Metals, Vol. 56, No.7 (2006), pp.376-385) by Sakaki, it is described that “in cold spray, gas, the temperature of which is lower than the melting point or the softening temperature of powder material, flows in a Laval nozzle (or a convergent-divergent nozzle) so as to make a hypersonic flow, as shown in FIG. 2. Then, the particulate material is accelerated by being poured into the hypersonic flow, and the solid-state particulate material collides with a substrate at high speed so as to form a coating on the substrate.”
In “Mechanical Properties of Aluminum Coatings Produced by Cold Spraying” (Journal of the Society of Materials Science, Japan, Vol. 56, No.6, pp.550-555, June 2007) by Itoh et al., it is described that “the density of a cold sprayed coating is slightly lower than that of a pure milled aluminum, and the porosity of the cold sprayed coating is 2.52%. The density of an atmosphere plasma-sprayed coating is lower than that of the cold sprayed coating, and the porosity of the plasma-sprayed coating is 7.49%. The Young's modulus of the cold sprayed coating is significantly larger than that of the plasma-sprayed coating, and is generally equal to 80% of the Young's modulus of the pure milled aluminum.”
In the thermal spraying of the prior art, the conductor must be instantly cooled after being melted. However, it is difficult to control the rate of cooling. Depending on the rate of cooling, the grain size of the conductor may be excessively increased (in other words, annealing occurs). Such increased grain size may cause degradation of the formed conductor. As a result, the conductive end ring cannot have sufficient strength, and the rotor having such an end ring is not reliable.